The present invention relates to a piezoelectric actuator which has a piezoelectric module constructed by laminating a plurality of piezoelectric elements.
A piezoelectric material is characterized in that it expands and contracts with high speed response. Therefore, the material can be used for an actuator which requires high speed mechanical movement. This kind of actuator is generally constructed as shown in FIG. 5. Namely, the actuator has a piezoelectric module 5 in which a plurality of piezoelectric elements 1 are laminated together with electrode plates 3 sandwiched therebetween. This type of actuator is disclosed in Japanese Published Unexamined patent applications Nos. 60-39878 and 60-39879. In the piezoelectric module 5, as shown in FIG. 6, one alternate group of electrode plates 3 are connected to a positive electrode (not shown), and the other group of electrode plates 3 are connected to the negative electrode (not shown). One end 5a of the piezoelectric module 5 is fixed to a housing of the actuator. Upon application of an electrical charge, each piezoelectric element 1 expands or contracts so that another end 5b of the module 5 moves, thereby providing mechanical displacement. As shown in FIG. 6, in the prior art, all of the laminated piezoelectric elements 1 are unified in shape and cross section (in relation to the laminating direction).
However, the prior art piezoelectric actuator has certain drawbacks which will be explained below. When the piezoelectric module expands or contracts, each piezoelectric element 1 receives an inertial resistance load from the elements proximate the movable end 5b. If a large load and displacement are required, e.g., if the actuator is used for a fuel injection valve of a motor vehicle, the module is formed by laminating many piezoelectric elements together and is driven at a high frequency in order to attain high responsiveness. In this case, the piezoelectric elements 1 near the fixed end 5a receive an excessive inertial load, thereby damaging the elements.